The present application claims priority to Japanese Application(s) No(s). P2002-230136 filed Aug. 7, 2002, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a negative electrode active material containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl, a method of producing the same, and a nonaqueous electrolyte cell using the negative electrode active material.
Attendant on the reduction in size of electronic apparatus, there has been a demand for development of a secondary cell having a high energy density. As a cell meeting the demand, there is a lithium secondary cell. However, in the lithium secondary cell, there is the problem that dendritic precipitation of lithium occurs on the negative electrode at the time of charging, to make the negative electrode inactive, resulting in a short cycle life.
As a means for improving the charging-discharging cycle characteristic of the lithium secondary cell, the so-called lithium ion secondary cell has been commercialized. For the negative electrode in the lithium ion secondary cell, there has been used a graphite material utilizing the intercalation reaction of lithium between graphite layers or a carbonaceous material applying the doping-dedoping actions of lithium into pores. Therefore, in the lithium ion secondary cell, the dendritic precipitation of lithium does not occur, and the cycle life is longer. In addition, since the graphite material and the carbonaceous material are stable in air, the lithium ion secondary cell has a great merit also from the viewpoint of industrial production.
However, the capacity of the negative electrode utilizing the intercalation has an upper limit determined by C6Li, which is the composition of the first stage inter-graphite-layer compound. Besides, control of the fine porous structure of the carbonaceous material is difficult to achieve industrially and leads to a lowering in the specific gravity of the carbonaceous material, so that this approach cannot be an effective means of enhancing the negative electrode capacity per unit volume and, hence, the cell capacity per unit volume. It is known that a certain kind of carbonaceous material shows a negative electrode discharging capacity in excess of 1000 mAh/g. However, in the case where a cell including a metallic oxide or the like as the positive electrode is constituted for achieving a great capacity at a noble potential of not less than 0.8 V against lithium metal, there is the problem that the discharging voltage is lowered or the like problem.
For the above reasons, it is considered difficult for the negative electrode active material using the carbonaceous material in the present situation to cope with the prolongation of the period of time for which the electronic apparatus is to be used and the enhancement of energy density of the power source. Thus, there is a demand for a negative electrode active material which has a further higher doping-dedoping ability.
In view of the above-mentioned demand, materials capable of alloying with lithium, such as Zn, Cd, Pb, Sn, Bi, Si, In, Sb, Ge, etc. have been widely studied as negative electrode active material for attaining a high-capacity negative electrode. In addition, Lixe2x80x94Al alloys, Lixe2x80x94Si alloys as disclosed in U.S. Pat. No. 4,950,566, and the like have also been studied. Besides, negative electrode active materials using a compound of a Group 4B element other than carbon which contains at least one non-metallic element are disclosed in Japanese Patent Laid-open No. Hei 11-102705.
However, such materials as Zn, Cd, Pb, Sn, Bi, Si, In, Sb, Ge, etc., the Lixe2x80x94Al alloys, the Lixe2x80x94Si alloys and the like, and the compounds of a Group 4B element other than carbon which contains at least one non-metallic element all have the property of expanding and contracting attendant on the doping and dedoping of lithium, so that the cells using such a material as the negative electrode active material have the inconvenience that upon repeated charging and discharging the negative electrode becomes particulate, with the result of a conspicuous degradation of the charging-discharging cycle characteristics.
In order to improve the charging-discharging cycle characteristics, a method of adding to the negative electrode active material an element irrelevant to the expansion and contraction attendant on the doping and dedoping of lithium and the like methods have been investigated. For example, Japanese Patent Laid-open No. Hei 6-325765 discloses LixSiOy (xxe2x89xa70; 2 greater than y greater than 0), Japanese Patent Laid-open No. Hei 7-230800 discloses LixSi1xe2x88x92yMyOz (xxe2x89xa70; 1 greater than y greater than 0; 2 greater than z greater than 0), and Japanese Patent Laid-open No. Hei 7-288130 discloses Lixe2x80x94Agxe2x80x94Te alloys.
However, the improvements of the degradation of the charging-discharging cycle characteristic arising from the expansion and contraction of the alloy by these methods are still insufficient, and the characteristic features of the alloys have not yet been made most of.
The present invention has been made for solving the above-mentioned problems in the related art. Accordingly, it is an object of the present invention to provide a negative electrode active material capable of restraining the expansion and contraction attendant on the doping and dedoping of lithium which is characteristic of the alloy material, and a method of producing the same. In addition, it is another object of the present invention to provide a nonaqueous electrolyte cell which can restrain the change of the negative electrode into a particulate form attendant on the charging and discharging and can simultaneously show both high charging-discharging cycle characteristic and a high discharging capacity.
According to the first aspect of the present invention, there is provided a negative electrode active material prepared by subjecting to a mechanical milling treatment an alloy powder containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl.
According to the second aspect of the present invention, there is provided a negative electrode active material prepared by subjecting a raw material including a powder containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl to a mechanical alloying treatment at a reaction temperature of below 90xc2x0 C.
According to the third aspect of the present invention, there is provided a method of producing a negative electrode active material which includes subjecting to a mechanical milling treatment an alloy powder containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl.
According to the fourth aspect of the present invention, there is provided a method of producing a negative electrode active material which includes subjecting a raw material including a powder containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl to a mechanical alloying treatment at a reaction temperature of below 90xc2x0 C.
According to the fifth aspect of the present invention, there is provided a nonaqueous electrolyte cell including a negative electrode including a negative electrode active material, a positive electrode, and a nonaqueous electrolyte, wherein
the negative electrode active material is prepared by subjecting to a mechanical milling treatment an alloy powder containing at least one element selected-from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl.
According to the sixth aspect of the present invention, there is provided a nonaqueous electrolyte cell including a negative electrode including a negative electrode active material, a positive electrode, and a nonaqueous electrolyte, wherein
the negative electrode active material is prepared by subjecting a raw material including a powder containing at least one element selected from the group consisting of the Group 14 elements exclusive of C and the Group 13 elements exclusive of Tl to a mechanical alloying treatment at a reaction temperature of below 90xc2x0 C.
According to the present invention, the alloy powder is changed in shape by the mechanical milling treatment, whereby a negative electrode active material restrained from the expansion and contraction attendant on the doping and dedoping of lithium can be obtained.
According to the present invention, the reaction temperature is controlled to be below 90xc2x0 C. in subjecting the powder as the raw material to the mechanical alloying treatment, the expansion and contraction attendant on the doping and dedoping of lithium can be restrained.
In a method of producing a negative electrode active material according to the present invention, the alloy powder is changed in shape by the mechanical milling treatment, whereby a negative electrode active material restrained from the expansion and contraction attendant on the doping and dedoping of lithium can be obtained. Further, by using this negative electrode active material, a nonaqueous electrolyte cell having a greatly enhanced charging-discharging cycle characteristic and a high discharging capacity can be realized.
In a method of producing a negative electrode active material according to the present invention, the reaction temperature is controlled to be below 90xc2x0 C. in subjecting the powder as the raw material to the mechanical alloying treatment, the expansion and contraction attendant on the doping and dedoping of lithium can be restrained. Further, by using this negative electrode active material, a nonaqueous electrolyte cell having a greatly enhanced charging-discharging cycle characteristic and a high discharging capacity can be realized.
The above and other objects, features and advantages of the present invention will become-apparent from the following description and appended claims, taken in conjunction with the accompanying drawings which show by way of example some preferred embodiments of the invention.